I don’t know to start looking for how to do this; eq might be what I need, but I don’t know how to mix that with slice and taxonomies.
I use this to exclude all taxonomies from my sitemap.xml:
{{- $exclude := slice “taxonomy” -}}
{{ if not (in $exclude .Kind) }}
My taxonomies are in config.toml:
[taxonomies]
tag = “tags”
category = “categories”
author = “authors”
But now I want to include authors in the sitemap. How can I exclude all taxonomies - like tags and categories - except for authors? Or, only include the taxonomies I want?
Have a look at this example: https://gohugo.io/functions/union/#or-filter-in-where-query
Edit:
For using in an if statement: or
{{ $include := slice "authors" }}
{{ if (in $include .Type) }}
should implicitly include only the ones you want
Thanks, that works and includes only authors. But that now excludes all my other content, like posts and pages. 
I tried what I think is “if is a taxonomy, then select only authors”, but this doesn’t work:
{{ $taxonomy := .Site.Taxonomies }}
{{ if (in $taxonomy .Type) }}
{{ $include := slice “authors” }}
{{ if (in $include .Type) }}
{{ end }}
{{ $in := slice "authors" }}
{{ if or (in $in .Type) (not (eq .Kind "taxonomy")) }}
true: ^ if authors OR ^ not a taxonomy .Kind
Thanks! That works great.